The present invention is directed toward methods for polymerizing compositions in-situ using a radiation source such that polymer films are formed therefrom.
Polymeric materials (also referred to herein simply as “polymers”) are used in many applications. Polymeric materials can be formed into a wide variety of shapes suitable for their intended application. Some applications impose more stringent requirements on dimensions or other properties of materials used than others. For example, optical clarity of polymeric materials is an important consideration when selecting polymeric materials for use in optical applications. As a further example, many applications require that polymeric materials used therein consist of single layer films having controlled dimensions.
A “film” is generally understood to be a relatively thin, continuous, single layer of material. In contrast, many conventionally applied “coatings” do not form a continuous or uniform layer of material on an underlying substrate. As such, coatings (e.g., vapor coatings and ink jet-printed coatings) are often not able to be physically separated from the supporting substrate on which they are formed so that they can be used as a stand-alone layer or as one of multiple layers in another application. Thus, such coating technology has its limitations and is generally deficient for formation of polymeric films.
U.S. Pat. No. 4,207,356 describes one application of coating technology. Disclosed therein is a method of coating glass containers with a layer of plastic. Using the methods therein, uncured polyurethane liquid plastic is mixed and metered in predetermined amounts to each nozzle means and cast from separate nozzle means as the bottle is rotated at approximately 40-60 revolutions per minute beneath the nozzle means. According to the methods therein, liquid plastic is said to be flow-coated to a thickness of 100-250 microns per bottle, sometimes using multiple passes to obtain the desired coating thickness. After the coating is cast onto the bottle, the coated bottle is moved to a curing zone for a curing step. Curing can be accomplished using thermal radiation or by photocuring.
Similarly, U.S. Pat. No. 4,034,708 describes a casting operation for making plastic emblems. As described therein, an operator applies measured portions of a plastic material, such as liquid polyurethane, to the upper surface of a substrate. Preferred are polyurethane resins to which a catalyst is added just prior to casting in order to initiate a curing reaction. Further, an infrared radiation source may be provided to irradiate the polyurethane in order to rapidly drive off volatile liquids present in the liquid polyurethane and promote curing of the composition. Also discussed therein is the possibility of irradiating the substrates prior to casting, thereby reducing the viscosity of the cast plastic as it flows onto the substrate. This is said to allow for more even flow over larger substrates. Whatever method is used, however, an objective discussed throughout this patent is the prevention of plastic material from flowing over the edge of the emblem on which it is applied; thus, it is stated that it is important to hold the substrate flat during the entire casting and curing process.
See also U.S. Patent Publication No. 2003/0148044, which discusses plastic emblems having an enhanced depth of vision. In addition to a layer of plastic material therein, such emblems contain a transparent plastic overlay flow-coated over the image or design therein. It is stated that one preferred plastic material is polyurethane comprising the reaction product of: (A) a polyester glycol and low to medium molecular weight polypropylene triols, and (B) aliphatic diisocyanatepolypropylenetriol adduct. After mixing (A) and (B), the mixture is cast onto a decorative substrate to form a radiused edge based on the flow pattern, after which time it is cured by heat or irradiation such as ultraviolet irradiation. Gel times of such polyurethanes are selected to be approximately 4 minutes to 7 minutes. It is stated that preferably a catalyst is added to component (A) in order to promote a slow cure at room temperature so as to allow full flow of the liquid polyurethane to the edges of the substrate before setting.
Also see U.S. Pat. No. 6,045,864, which describes a coating system and method that allows coatings, such as polyurethane coatings, to be formed from a variety of coatable compositions that are entirely free of or have relatively little solvent. A fluid composition described therein is atomized and contacted with a carrier gas to vaporize substantially all of the atomized fluid composition, which condenses onto a surface to form a coating. However, such coatings are said to be capable of formation to thicknesses ranging only from 0.01 micrometer to 5 micrometers in a single pass, requiring multiple depositions or passes for formation of thicker films or multilayer sheets.
Due to the limitations of coating technology, many coating layers so formed are used in combination with one or more other layers in various articles. For example, see U.S. Pat. No. 7,160,973, which describes preparation of urethane polymers. The polymers are said to be useful as coating compositions that can be applied on an article/substrate by techniques that include spray coating, dip coating, roll coating, curtain coating, and the like. The coating compositions are also stated to be useful as a high-gloss coating and/or as the clearcoat of a composite color-plus-clear coating.
As a further example, see U.S. Patent Publication No. 2009/0297724, which describes an ultraviolet-curable coating composition containing aliphatic urethane resins. Application of the compositions therein as a “top coating” having a thickness of about 3 to about 35 microns on a base plastic material having a thickness of about 0.75 to 20 mm is described. A cured such top coating is stated to provide a high degree of scratch, abrasion, mar and chemical resistance along with superior UV resistance, exterior durability and thermal stability. The coating is described as being applied by various conventional coating methods, such as spray coat, rotary atomization, flowcoat, curtain coat, or roll coat techniques to a film thickness of about 3 microns to about 40 microns, with the most preferred dry film thickness being about 5 microns to about 20 microns. Coatings therein are described as being useful for exterior protection on automobiles.
In contrast to coatings, a film may be used apart from an underlying substrate on which it is typically formed. Further, films are capable of imparting desired properties to their intended application without the need for coating multiple layers or laminating multiple films together.
Polymeric films are widely used in many applications. Whether a polymeric film is suitable for an intended application depends upon, for example, its physical properties such as strength, elasticity, clarity, color, durability, and the like. To be desirable for use in an application, however, preparation and application of the polymeric film must be cost-effective.
In addition to minimization of cost, optimization of a film for an intended application poses other challenges. In regards to optical applications, the amount of gelation occurring during formation of the film has been found to impact its optical qualities. A “gel” is generally understood to be a viscous composition, which in polymer processing can be, for example, an at least partially polymerized composition, one having a relatively high molecular weight, and/or one containing significant amounts of entrapped gas (e.g., air or reaction by-products, such as carbon dioxide). Gelation can make formation of uniform layers of polymeric material (e.g., films) difficult. Hence, optical quality of a film formed in the presence of significant gelation is often compromised.
Although “non-yellowing” films advertised as having “low gel” content are known (e.g., ARGOTEC 49510, a polycaprolactone based, aliphatic polyurethane film available from Argotec, Inc. of Greenfield, Mass.), preparation of such films is difficult. For example, a contributing cause to deterioration in a film's optical quality is gelation associated with entrapment of gases. Entrapment of gas, such as carbon dioxide produced when conventionally processing polyurethane films, is often encountered when polymerizing materials. The entrapped gas creates imperfections in the material, which can appear as visible imperfections impairing the optical qualities of the material. In optical applications, imperfections having a size of about 10 microns or greater are generally objectionable. Imperfections having a size of as small as about 5 microns are even often viewed as problematic.
Gelation also complicates the common hot-melt processing of polymeric materials. For example, when processing a conventional hot-melt processable composition into a film format, polymerization of the composition causes gelation that can result in processing inefficiencies in that, e.g., dispensing nozzle or extrusion, equipment used therewith can become clogged due to the continuously increasing or non-uniformly increasing viscosity of the polymerizing composition during hot-melt processing thereof.
In addition to the disadvantages associated with gelation in conventional processing of polymeric materials, many conventional processing techniques lack the overall processing efficiency desired. For example, a further disadvantage of conventional hot-melt processing techniques relates to the fact that hot-melt processing generally involves multiple processing steps. For example, in many applications, some method of increasing the cohesive strength of applied hot-melt compositions is often required (e.g., post-crosslinking or moisture-curing). In addition, some polymer chemistries are not capable of being hot-melt processed due to their relatively high molecular weight and/or the presence of crosslinking, both of which can make it difficult, if not impossible, to hot-melt process preformed pellets of the compositions (as is done generally according to conventional methods of forming polymer films using hot-melt processing) at a temperature below the degradation temperature of the polymer composition or substrate onto which the film is formed. Thus, the properties of conventionally manufactured polymer films are limited to those particular polymer chemistries that can be formed into films using conventional methodology.
Alternative methods to hot-melt processing also have their disadvantages, including not only the need to often perform additional processing steps to increase the cohesive strength of the material after film formation, but other disadvantages as well. In addition to essentially 100% solid hot-melt systems, it is known to produce polymers in both solventborne (i.e., those using mostly organic solvents as a solvating medium) and waterborne (i.e., those using mostly water as a dispersing medium) systems. These systems are applied to a substrate in the form of a solution or dispersion, respectively. Whether the system is solventborne or waterborne, however, it must first be coated onto a desired substrate and then dried to remove solvating or dispersing medium (i.e., organic solvent or water, respectively) in order to form a polymer film. Thus, processing efficiency is compromised by these additional processing steps, much as processing efficiency is compromised by the need to otherwise cure a composition after coating it onto a substrate using hot-melt processing or otherwise. In addition, formation of polymer films of sufficient thickness can be problematic using these alternative methods. Further, organic solvent-based polymerization methods present environmental concerns and are typically costly to utilize. In addition, some polymer chemistries are not capable of being formed into polymer films using solventborne methods due to the lack of adequate solubility of such polymers or their constituents in conventional solvents.
Still further, while continuous methods of polymerization on a web are known (i.e., conventional methods of on-web polymerization of (meth)acrylate adhesives), those methods typically require additional processing steps as well. For example, processing steps associated with pre-polymerizing conventional compositions to increase their viscosity such that the compositions are coatable onto the web are generally required when using such methods. If such pre-polymerization is not performed, the generally low molecular weight monomers used in preparation of such adhesive films typically flow uncontrollably off the web onto which they are coated before being polymerized. When performing such pre-polymerization, however, process efficiency is compromised as such pre-polymerization generally requires the use of an expensive chemical reactor or obtainment of specialized components that are pre-polymerized. Thus, alternative processing methods are desirable to improve overall efficiency when processing polymeric materials.
It is known to use a variety of processes for formation of articles having polymer (e.g., polyurethane-based) layers and systems utilizing a variety of chemistries in order to improve overall performance properties of the polymer system. In addition to the variety of conventional processing techniques for formation of conventional polymer films and articles, a wide variety of polymer chemistries are known. Polymer chemistry is often selected according to the intended end-use application.
Polyurethane-based chemistries are well known and used in many different types of applications. For example, polyurethane-based chemistries are known for their ability to provide superior optical and other performance properties. Despite the widespread use of polyurethane-based chemistry, obtainment of both maximized optical performance and processing efficiency has often not been possible when using traditional methods for processing polyurethane-based films.
Thus, polyvinyl chloride is often used as a less expensive substitute for polyurethane-based chemistries, particularly in cost-sensitive applications (as the cost of polyvinyl chloride films is often about 10% to about 35% of the cost of a comparable polyurethane film). However, polyvinyl chloride is less desirable than polyurethane-based chemistries because of, for example, problems associated with plasticizer migration inherent when using polyvinyl chloride (that is of sufficient flexibility for many applications) and the controversial use of vinyl chloride monomers. Commonly used polyvinyl chloride plasticizers (e.g., phthalates) have been shown to negatively affect certain hormonal functions such as a body's endocrine system. Further, vinyl chloride monomers have been recognized as a carcinogen since the early 1970s. Still further, when polyvinyl chloride is burned, it often creates hazardous, halogen-based air pollutants such as hydrogen chloride. As such, many environmental and public safety organizations strongly oppose the manufacture of polyvinyl chloride, especially plasticized polyvinyl chloride, many governments are considering legislating or banning the use of polyvinyl chloride, and many companies are phasing out the use of polyvinyl chloride in their products.
Not surprisingly, alternatives to conventional polyvinyl chloride, such as increased use of polyurethane chemistry, are desirable and of interest. For example, U.S. Pat. No. 5,428,087 describes preparation of a modified polyvinyl chloride composition using blocked isocyanate and polyol and/or polyamine components that react upon heating to form a polyurethane or polyurea polymer network in-situ within the gelling polyvinyl chloride composition. The isocyanate component is said to contain isocyanate groups that must be blocked to enable the composition to be produced as a single component final product at a first location and then transported to a second location for actual article-forming use many days later without fear of premature gelation/network formation. The addition of the polyurethane or polyurea network is said to increase the resistance of the polyvinyl chloride to heat and solvents and reduce the occurrence of plasticizer migration therein.
Similarly, U.S. Pat. No. 7,157,527 describes preparation of interpenetrating polymer networks using blocked polyurethane/polyurea prepolymers. The polymer networks formed are based on a polyurethane or polyurea prepolymer in combination with a polymeric component including an acrylate resin or epoxy resin. The networks so formed are said to be useful as layers in golf balls.
U.S. Pat. No. 7,138,466 describes a polyurethane hot-melt adhesive composition. The adhesive composition is prepared using a moisture curable reactive hot-melt process. The compositions therein are said to have improved green strength and be useful for bonding a number of articles.
U.S. Pat. No. 4,292,827 describes a method for making decorative emblems, plaques, or panels comprising flow coating a clear, fluent plastic material (i.e., stated to be preferably a fluent polyurethane) onto the surface of a decorated substrate. Flow coating is accomplished with a multiple orifice nozzle(s) that is passed over the surface of the decorative substrate at a steady speed to give a uniform coating thickness of 0.020 inch to 0.030 inch. The flow-coated plastic is then cured and the coated, decorative substrate is stamped to form slightly convex emblems, plaques, or panels. Similarly, U.S. Pat. No. 4,332,074 describes formation of an integral bezel around the periphery of such a decorative surface.
While polyurethane-based chemistry is used to form a number of useful polymer films and articles, the use of such chemistry has not been successfully expanded to enable efficient formation of films. Thus, alternative methods for formation of polymer films, particularly polyurethane-based films, are desired. It would also be advantageous to provide polymerizable compositions that are polymerizable to films using improved methods, including continuous methods. Further, there is a recognized need to improve not only processing efficiency, but also optical properties of polymer films so formed.